What error by the Confederates led to the occupation of Nashville by Union troops?

Answer 1

Locating Fort Henry and Fort Donelson in poorly chosen terrain led to their easy capture by Union forces, precipitating Confederate withdrawal from about two-thirds of Tennessee. These Forts, situated on the Tennessee and Cumberland Rivers, were placed right at the Kentucky state line when they were begun, out of respect for the neutrality Kentucky was proclaiming at the time work was commenced. Unfortunately for the Confederates those places were very low ground, and prone to flooding, easily invested by attackers. Armored Union gunboats could sail practically up to the gates. These two Rivers were important because, unlike the rivers in the east which flowed generally west to east, these two had a north-south orientation. With the superior naval capabilities of the north, they were like daggers poised at the Rebel heartland. The danger was appreciated, hence the Forts built on each. But a spot on a high bluff commanding the Rivers for some distance would have been an infinitely better choice, even though not protecting every inch of Tennessee territory.